1. Field of the Invention
The present invention relates to a display element. More specifically, it relates to a display element for repeatedly displaying an image by driving colored particles with an electric field.
2. Description of the Related Art
Examples of display element having excellent holding property and capable of being rewritten repeatedly include twisting ball displays (two-color painted particle rotating display), electrophoresis display mediums, magnetophoresis display mediums, thermal rewritable display mediums, liquid crystals having a memory property, or the like. However, since these elements cannot display a white color like paper, they have a problem of low image contrast.
In order to solve the low contrast problem, display techniques using toners have been proposed (Japan Hardcopy, ""99 Article Collection, pp. 249-252, and Japan Hardcopy, Fall Preliminary Report Collection, pp. 10-13). According to the display technique, a conductive colored toner and a white particle are sealed in a space between a display substrate and a non-display substrate, which face each other. The display substrate and the non-display substrate each comprise an electrode. A charge is injected to the conductive colored toner via a charge conveying layer provided in the electrode of the non-display substrate. The conductive colored toner into which the charge was injected is moved to a side of the display substrate disposed facing the non-display substrate by the electric field therebetween. By adhesion of the conductive colored toner to the display substrate, the color of the conductive colored toner is displayed on the display substrate. Accordingly, a high contrast image is displayed with the color of the conductive colored toner and the white of the white particle.
According to the display technique, since the solid colored toners and white particle are used, unlike the case of using a liquid instead of either one of the toner and the particle, deterioration of the display density due to intrusion of the liquid into the toner or the particle does not occur. Therefore, theoretically the display can be switched completely between the white and the toner color (for example, black). However, according to this display technique, a charge is not injected to a toner, which does not contact the charge conveying layer provided in the electrode of the non-display substrate and is isolated from the other conductive colored toners. The toner without the charge injection exists in a space between the substrates randomly without being moved by the electric field so that the toner and the white particles exist in a mixed manner, and thus a problem of the image display with a low contrast is involved in practice.
A method for forming an image by moving or rotating a particle that was charged by the effect of the electric field, is proposed in the official gazette of the Japanese Patent Application Laid-Open (JP-A) No. 2001-33833. According to the method, substrates provided with a conductive layer or a rectifying layer are disposed facing each other, and two kinds of colored particles having colors different from each other and without a charge of the same polarity are sealed between the substrates. By transferring a charge to each particle from the conductive layer or the rectifying layer and moving or rotating the particles by the effect of the electric field, the image is displayed. However, the method has a problem of image density irregularities and contrast deterioration being generated due to local charge leakage.
An image display medium comprising a pair of substrates and a group of different kinds of particles having different colors and charge characteristics, sealed between the substrates, movably between the substrates by an applied electric field is proposed in the official gazette of the Japanese Patent Application No. 2000-165138. According to this image display medium, high whiteness degree and contrast can be obtained. However, the driving voltage is increased, because the image display medium requires an applied voltage of several hundred volts for displaying an image, therefore, this image display medium has a problem that the degree of freedom in designing the driving circuit is lowered.
Moreover, with repetition of contact electrification of the particles and the substrate, and frictional electrification among the particles, the amount of the charge of the particles becomes too large relative to an initial charge amount, and the particles coagulate or adhere to the substrate. As a result, there are also problems of the contrast being reduced, the unevenness in image density, and the driving voltage increasing.
Accordingly, in order to solve the above-mentioned problems, the present invention has been achieved, and an object thereof is to provide a display element capable of displaying an image with a high contrast and few density irregularities, which is driven at a low voltage, and has an improved image holding property.
In order to solve the above-mentioned problems, a first aspect of the present invention provides a display element comprising a first substrate and a second substrate, which are disposed facing each other, and multiple kinds of particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate and can be moved between the first substrate and the second substrate by an electric field applied thereto, wherein the surface coarseness of at least one of the first substrate and the second substrate, on the surface thereof that faces the other substrate, is more than 1 nm and no greater than 10 xcexcm, and the coefficient of a shape of the particles is more than 100 and no greater than 140 when FE represents the coefficient of shape of the particles, L represents the circumference of the particles, S represents the area of the particles and FE=((L2/S)/4xcfx80)xc3x97100.
According to the first aspect of the present invention, when the surface coarseness of at least the first substrate and the second substrate on the surface of the side facing the other substrate is more than 1 nm and 10 xcexcm or less, and the coefficient of shape of the particles is more than 100 and 140 or less (wherein FE is the coefficient of shape of the particles, L is the circumference of the particles, S is the area of the particles and FE=((L2/S)/4xcfx80)xc3x97100), the distance between the substrates and the particles becomes longer and the contact area of the substrates and the particles becomes smaller, and thus the particles adhered to the substrate can be separated easily. Thus, the driving voltage of the display element can be reduced. In this case, the coefficient of shape FE represents the roundness of the particles are projected onto a plane, and the circumference L and the area S each represent a value measured by projecting the particles onto a plane.
A second aspect of the present invention provides a display element comprising a first substrate and a second substrate disposed facing each other, and multiple kinds of substantially spherical particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate, and can be moved between the first substrate and the second substrate by an electric field, applied thereto, wherein the surface coarseness (Rz) of at least one of the first substrate and the second substrate is smaller than the average diameters of the multiple kinds of particles.
According to the second aspect of the present invention, since the surface coarseness (Rz) of at least one of the first substrate and the second substrate is smaller than the average diameters of the multiple kinds of particles, the particles cannot be fitted into the concave surface of the substrate and the particles adhered to the substrate can be separated easily. Accordingly, the driving voltage of the display element can be reduced.
A third aspect of the present invention provides a display element comprising a first substrate and a second substrate disposed facing each other, and multiple kinds of particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate and can be moved between the first substrate and the second substrate by an electric field applied thereto, wherein the water contact angle of at least one of the first substrate and the second substrate, on the surface thereof that faces the other substrate, is at least 70xc2x0.
According to the third aspect of the present invention, since the contact angle with respect to water of at least one of the first substrate and the second substrate on the surface of the side facing the other substrate is 70xc2x0 or more, the water repellent property can be improved so that the particles adhered on the substrate can be separated easily. Thus, the driving voltage of the display element can be reduced.
A fourth aspect of the present invention provides a display element comprising a first substrate and a second substrate disposed facing each other, and multiple kinds of particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate and can be moved between the first substrate and the second substrate by an electric field applied thereto, wherein the coefficient of dynamic friction of at least one of the first substrate and the second substrate, on the surface thereof that side faces the other substrate is no more than 0.7.
According to the fourth aspect of the present invention, since the coefficient of dynamic friction of at least one of the first substrate and the second substrate on the surface of the side facing the other substrate is 0.7 or less, the resistance between the substrate and the particles by the coefficient of dynamic friction can be reduced, and thus the particles adhered to the substrate can be separated easily. Thus, the driving voltage of the display element can be reduced.
A fifth aspect of the present invention provides a display element comprising a first substrate and a second substrate disposed facing each other, and multiple kinds of particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate and can be moved between the first substrate and the second substrate by an electric field applied thereto, wherein the surface resistance of at least one of the first substrate and the second substrate, on the surface thereof that faces the other substrate, is at least 1.0xc3x97107 xcexa9/cm2.
According to the fifth aspect of the present invention, since the surface resistance of at least one of the first substrate and the second substrate, on the surface of the side facing the other substrate is 1.0xc3x97107 xcexa9/cm2 or more, movement of the charge from the particles to the substrate is suppressed so that the charge of the particles can be maintained over a long time, thereby improving the image holding property. Thus an image with a high contrast can be displayed without image irregularities.
A sixth aspect of the present invention provides a display element comprising: a first substrate and a second substrate disposed facing each other, and multiple kinds of particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate and can be moved between the first substrate and the second substrate by an electric field applied thereto, wherein a gas barrier layer is provided on at least one of the first substrate and the second substrate.
According to the sixth aspect of the present invention, since a gas barrier layer is provided on at least one of the first substrate and the second substrate, the particles are shielded from the atmosphere so that the charge characteristics of the particles can be stable, and thus the image holding property can be improved and an image with a high contrast can be displayed without image irregularities.
A display element of the seventh embodiment comprises: a first substrate and a second substrate disposed facing each other; and multiple kinds of particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate and can be moved between the first substrate and the second substrate by an electric field applied thereto, wherein a surface layer, which has a volume resistivity value as expressed by a common logarithmic value (logxcexa9xc2x7cm) of 7 to 16, is disposed on a surface of at least one of the first substrate and the second substrate, which surface faces the other substrate.
According to the seventh aspect of the present invention, by at least one of the first substrate and second substrate having the surface layer on the surface thereof that faces the other substrate, and the volume resistivity of the surface layer, expressed by a common logarithmic value (logxcexa9xc2x7cm), being from 7 to 16, an electrification characteristic is stabilized over a long period of time and coagulation of the particles is suppressed. Thus, a high contrast image having no image unevenness can be displayed with low driving voltage and stable repetition.